Oscillators – Ring oscillators
Reexamination Certificate
2000-08-29
2002-06-11
Mis, David (Department: 2817)
Oscillators
Ring oscillators
C331S175000, C331S17700V, C331S185000
Reexamination Certificate
active
06404295
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a voltage controlled oscillator.
2. Description of the Related Art
Generally, a voltage controlled oscillator is employed in a phase locked loop (PLL) circuit to control the frequency of an oscillation signal. An exemplary circuitry arrangement is shown in FIG.
1
. As shown in
FIG. 1
, when the voltage level at an input terminal
11
is shifted from a first voltage of V to a second voltage V+&Dgr;V, the frequency of the oscillation signal outputted from an output terminal
12
varies from F to F+&Dgr;F.
In the conventional voltage controlled oscillator, the input terminal
11
is connected to the gate of an N-channel MOS (n-channel metal oxide semiconductor) transistor or a P-channel MOS (p-channel metal oxide semiconductor) transistor. Current supplied to a train of inverters is varied to control the oscillation signal frequency.
The frequency of the oscillation signal generated by a train of inverters
14
is proportional to the current flowing through a second P-channel MOS transistor
102
. The current flowing through the second P-channel MOS transistor
102
is proportional to the current flowing through a first P-channel MOS transistor
101
. The current flowing through the first P-channel MOS transistor
101
is equal to the current flowing through a first N-channel MOS
201
. When the voltage at the input terminal
11
is shifted from the ground level to the power supply voltage level, any current does not flow through the first N-channel MOS transistor
201
until the voltage at the input terminal
11
rises up to a threshold voltage of the first N-channel MOS transistor
201
.
When the voltage at the input terminal
11
exceeds the threshold voltage of the first N-channel MOS transistor
201
, the current flows through the first N-channel MOS transistor
201
. As the voltage at the input terminal
11
is increased, the current flowing through the first N-channel MOS transistor
201
increases. However, at this time, the voltage at the first terminal
13
decreases, so that the current is gradually saturated.
FIGS. 2A and 2B
show two examples of the inverter
14
used in the above voltage controlled oscillator.
FIG. 2A
shows an arrangement one inverter. In
FIG. 2A
, the inverter is provided between the power supply voltage
20
and the ground
21
. Reference numerals
22
and
23
denote an input and an output.
FIG. 2B
shows the inverter composed of an N-channel MOS transistor and a P-channel MOS transistor.
The relation between the input voltage V at the input terminal
11
and the frequency F of the oscillation signal outputted from the voltage controlled oscillator is as shown in FIG.
3
. As seen from
FIG. 3
, the voltage at the input terminal
11
is limited to a range of 0.8 to 2.5 V and the operation frequency range is up to 300 MHz. Also, a ratio &Dgr;F/&Dgr;V increases simply. On contrary, when the input terminal
11
is connected to the gate of the P-channel MOS transistor in the voltage controlled oscillator, the input terminal
11
is similarly limited, and the ratio &Dgr;F/&Dgr;V decreases simply.
It is assumed that when the voltage at the input terminal is varied, the frequency of the oscillation signal outputted from the output terminal changes in a range between the minimum, FMIN and the maximum, FMAX. Also, variations in the performance of the oscillator are derived from deviation in the manufacturing conditions of the oscillator, deviations caused based on change in the operating temperature and deviation caused based on the change of the power supply voltage of the oscillator.
In consideration of all types of the variations of the performance, the best condition is established in a case of a maximum of gain, and the worst condition is established in case of a minimum of gain. The normal condition is established with no deviation between the best condition and the worst condition. In this case, a frequency range between the minimum FMIN and the maximum FMAX is referred to as an operation frequency range. In the conventional voltage controlled oscillator, either or both of FMIN and FMAX of the oscillation frequency are increased or decreased depending on the deviations. Therefore, when the deviations of the performance are large, the operation frequency range becomes narrow.
Recently, a fine pattern technique is applied to the components of an oscillator, and the power supply voltage is decreased. Therefore, the variations of the performance increase so that the operation frequency range is undesirably narrowed. For widening the operation frequency-range, it is necessary to increase the gain &Dgr;F/&Dgr;V where &Dgr;F is a change in the frequency of the oscillation signal outputted from the output terminal and &Dgr;V is a change in the voltage at the input terminal.
However, since the input terminal of the voltage controlled oscillator receives an analog voltage, the increase of the gain may cause the fluctuation or jitter in the period of the oscillation signal outputted from the output terminal.
In conjunction with the above description, a voltage controlled oscillation circuit is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 9-312521). In this reference, the voltage controlled oscillation circuit is provided to have linear control voltage—oscillation frequency characteristic over a wide range while keeping noise endurance. For this purpose, an input control voltage VC is compensated for by a linear compensating circuit (2) which has a voltage conversion characteristic inverse to the control voltage—oscillation frequency characteristic of the voltage controlled oscillation circuit (3). The oscillation frequency of the voltage controlled oscillation circuit (3) is controlled based on the control voltage V′C obtained by the voltage converting operation.
Also, a voltage controlled oscillator having efficient process compensation is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 9-508762). In this reference, a voltage controlled oscillator (VCO) circuit gives an output signal having an output frequency with the least change to an optionally predetermined input control voltage regardless of a manufacturing process change, a temperature change and a power supply voltage change. The VCO circuit contains a multi-stage ring oscillator which contains a plurality of current depletion inverters connected in series. The VCO circuit uses a first current source to give a substantially constant current independent from a process change, a temperature change and a power supply voltage change, and a second current source to give a variable current in response to the process change, the temperature change and the power supply voltage change. Both of the current sources generate current signals independent from the input signal to the VCO circuit. An attenuator gives a control current signal to the ring oscillator in response to an input voltage signal from a phase locked loop filter to the VCO circuit. The attenuator receives a supply current signal produced by subtracting the second current signal from the first current signal, and use a difference auxiliary circuit to attenuate the supply current signal in response to an input voltage to generate a control current signal which sets a current level of the cells of the ring oscillator. The oscillation frequency of the ring oscillator is determined based on the control current signal. Moreover, the VCO circuit includes a current mirror circuit to receive the control current signal from the attenuator and to reflect the control current signal to the ring oscillator.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a voltage controlled oscillator which can have a wide operation frequency range.
In order to achieve an aspect of the present invention, a voltage controlled oscillator includes a first converter, a second converter and an oscillator. The first converter outputs a first current proportional to an input voltage. In this case, an i
Minami Koichiro
Mizuno Masayuki
Mis David
NEC Corporation
Young & Thompson
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